Cathode active material and secondary battery comprising the same

ABSTRACT

Disclosed are a cathode active material and a secondary battery including the same, wherein the cathode active material includes (a) a first lithium-containing metal composite oxide and (b) a second lithium-containing metal composite oxide coated on an entire particle surface of the first lithium-containing metal composite oxide particle, the second lithium-containing metal composite oxide having a higher resistance and a lower potential vs. lithium potential (Li/Li+) than the first lithium-containing metal composite oxide.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of application Ser. No.12/523,392 which is a national phase entry under 35 U.S.C. §371 ofInternational Application No. PCT/KR2008/000305, filed Jan. 18, 2008,which claims priority from Korean Patent Application No.10-2007-0005568, filed Jan. 18, 2007 and Korean Patent Application No.10-2007-0022440, filed Mar. 7, 2007. The disclosures of saidapplications are incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a cathode active material and asecondary battery comprising the same, which can suppress the temporaryflow of a large amount of electrons and lithium ions during an internalshort, thereby improving safety.

BACKGROUND ART

Recently, as electronic devices have been miniaturized, and lightened,and portable electronic devices have been widely used, research on alithium secondary battery of high energy density has been activelyperformed. In a lithium secondary battery, materials allowingintercalation and deintercalation of lithium ions are used as an anodeand a cathode, and organic electrolyte or polymer electrolyte is chargedbetween the cathode and the anode. Also, when lithium ions areintercalated and deintercalated at the cathode and the anode,oxidation/reduction reactions generate electrical energy.

In a lithium secondary battery, a lithium-containing metal compositeoxide having a high potential vs. lithium potential (Li/Li⁺) (forexample, LiCoO₂) has been mainly used as a cathode active material, anda carbonaceous material has been mainly used as an anode activematerial. Such materials enable the lithium secondary battery to havehigh capacity and high power. However, when the above described cathodeactive material having a high potential vs. lithium potential (Li/Li⁺)is used alone, electrons and lithium ions suddenly move from an anode toa cathode during an internal short caused by internal/externalconditions. Accordingly, a large amount of current temporarily flows,and thus heat is suddenly generated within the battery, thereby causingfire or explosion of the battery.

In addition, when the cathode active material is exposed at a hightemperature atmosphere or a battery temperature is increased by anabnormal operation of a battery, the cathode active material isdecomposed at temperatures higher than a certain temperature, and thusgenerates oxygen. Accordingly, fire or explosion may occur in thebattery. Also, when side reactions occur by contacting the cathodeactive material with nonaqueous electrolyte, an exothermic reaction maycause an explosion. Especially, when gas is generated within the batterydue to the side reactions, the possibility of battery explosion isincreased.

DISCLOSURE OF THE INVENTION

Therefore, the present inventors found that when an entire surface of afirst lithium-containing metal composite oxide is coated with a secondlithium-containing metal composite oxide having a higher resistance anda lower potential vs. lithium potential than the firstlithium-containing metal composite oxide, during an internal short, itis possible to slow down the moving rate of a large amount of lithiumions and electrons from an anode to a cathode, and thus to prevent heatgeneration caused by the temporary occurrence of over-current, and toimprove safety of a battery. Accordingly, the surface of the firstcathode active material is stabilized, and thus it is possible toincrease the temperature where the first cathode active material isdecomposed. Also, since the first cathode active material is not incontact with electrolyte, the side reactions are inhibited, and theignition temperature is increased. Therefore, it is possible to improvethermal stability of a battery.

According to an aspect of the present invention, there is provided acathode active material comprising (a) a first lithium-containing metalcomposite oxide and (b) a second lithium-containing metal compositeoxide coated on an entire particle surface of the firstlithium-containing metal composite oxide, the second lithium-containingmetal composite oxide having a higher resistance and a lower potentialvs. lithium potential (Li/Li⁺) than the first lithium-containing metalcomposite oxide.

According to another aspect of the present invention, there is provideda secondary battery comprising a cathode, an anode, electrolyte, and aseparator, the cathode comprising the cathode active material.

Reference will now be made in detail to the exemplary embodiments of thepresent invention.

The present invention is characterized by a first lithium-containingmetal composite oxide of which the entire surface is coated with asecond lithium-containing metal composite oxide, wherein the secondlithium-containing metal composite oxide has a lower potential vs.lithium potential (Li/Li⁺) and a higher resistance than the firstlithium-containing metal composite oxide.

According to experimental examples, at a discharge rate of less than 5C, a first lithium-containing metal composite oxide, the entire surfaceof which is coated with a second lithium-containing metal compositeoxide of a high resistance value, shows a similar discharge graph to anuncoated first lithium-containing metal composite oxide (see FIGS. 4 and5). On the other hand, when the current rate is suddenly increased up tomore than 5 C, the coated first lithium-containing metal composite oxideshows a different discharge graph from the uncoated firstlithium-containing metal composite oxide (see FIGS. 6˜8). In otherwords, at a discharge rate of less than 5 C, when a firstlithium-containing metal composite oxide (which is firstly reduced dueto a high potential vs. lithium potential Li/Li⁺) is discharged, asecond lithium-containing metal composite oxide having a high resistancevalue does not act as a significant internal resistance, and on theother hand, at a discharge rate of more than 5 C, when the firstlithium-containing metal composite oxide is discharged, the secondlithium-containing metal composite oxide having a high resistance valueacts as a significant internal resistance.

During a discharge process, lithium ions are intercalated in a cathodeactive material, and herein, the cathode active material is reduced byreceiving electrons. Also, during a discharge process including aninternal short, a potential of a cathode is decreased, and lithium ions(Li⁺) and electrons are firstly intercalated in a firstlithium-containing metal composite oxide having a higher potential thana second lithium-containing metal composite oxide.

In the present invention, an entire surface of a firstlithium-containing metal composite oxide is coated with a secondlithium-containing metal composite oxide having a lower potential vs.lithium potential and a higher resistance than the firstlithium-containing metal composite oxide. Therefore, during a high-ratedischarge, such as an internal short, the second lithium-containingmetal composite oxide acts as a significant electric resistance layer,and suppresses the inflow of electrons into the core of the firstlithium-containing metal composite oxide having a high potential vs.lithium potential, thereby inhibiting the intercalation of lithium ions.In other words, during an internal short, it is possible to slow downthe intercalation rate of a large amount of lithium ions and electronsfrom an anode to a cathode, and thus to prevent heat generation causedby the temporary occurrence of over-current, thereby improving thesafety of a battery.

If only a part of the surface of a first lithium-containing metalcomposite oxide is coated with the second lithium-containing metalcomposite oxide, lithium ions and electrons may be intercalated in thefirst lithium-containing metal composite oxide through the uncoatedsurface. Accordingly, it is impossible to slow down the intercalationrate of lithium ions and electrons. Also, since a narrow surface,through which lithium ions and electrons pass, causes a nozzle effect, amoving rate of lithium ions and electrons is increased. Therefore, heatis locally generated, and thus may reduce the safety of a battery (seeFIG. 3).

Conventionally, in order to adjust a discharge potential of a cathodewithin a certain range, an electrode in which cathode active materialsof different potentials vs. lithium potential (Li/Li⁺) are included hasbeen used as a cathode for a lithium secondary battery. However, when aninternal short occurs, for the same reason described above, there hasbeen a problem in that fire or explosion of a battery may suddenlyoccur.

Meanwhile, in the present invention, since the surface of a firstlithium-containing metal composite oxide is uniformly coated with asecond lithium-containing metal composite oxide of a high potentialdifference and a high resistance, it is possible to maximize aresistance function and thus to suppress the flow of lithium ions whenover-current flows.

Contrary to an electrochemically irreversible electrode additive, whichhas been conventionally used for a specific effect, a cathode activematerial according to the present invention includes a secondlithium-containing metal composite oxide capable of intercalating anddeintercalating lithium. Accordingly, there are very few problems ofperformance degradation of a battery (such as, degradation of capacity,a lifetime property, etc.) which has been conventionally caused by anelectrode additive not concerned in the electrochemical reactions in abattery. In addition, since a first lithium-containing metal compositeoxide having a high potential vs. lithium potential, and a secondlithium-containing metal composite oxide having a relatively lowpotential vs. lithium potential, are used as a large amount of corematerial and a small amount of coating material, respectively, the firstlithium-containing metal composite oxide having a high drive voltage andhigh energy density can act as a main cathode active material.

Also, in the present invention, although a cathode active materialincludes a much smaller amount of second lithium-containing metalcomposite oxide than a different cathode active material in which afirst lithium-containing metal composite oxide and a secondlithium-containing metal composite oxide are simply mixed with eachother, it is possible to achieve the same or an even higher level ofimproved safety as the different cathode active material. Accordingly,it is possible to reduce the degradation of electrochemical properties,which may be caused by an excessive amount of a secondlithium-containing metal composite oxide.

Also, in the cathode active material according to the present invention,the surface of a first lithium-containing metal composite oxide iscompletely covered with a second lithium-containing metal compositeoxide. Accordingly, the surface energy of the first lithium-containingmetal composite oxide is lowered and stabilized, and thus it is possibleto increase a exothermic temperature where the first lithium-containingmetal composite oxide is decomposed and generates oxygen. Also, it ispossible to improve thermal stability by inhibiting side reactions ofthe first lithium-containing metal composite oxide and electrolyte.

The range of a potential difference vs. lithium potential between thefirst lithium-containing metal composite oxide and the secondlithium-containing metal composite oxide is not particularly limited.However, the first lithium-containing metal composite oxide preferablyhas a potential of 3.7 V or more (Li/Li⁺) in such a manner thatoxidation/reduction reactions of the first lithium-containing metalcomposite oxide can be performed at a relatively high potential. Also,the second lithium-containing metal composite oxide preferably has apotential vs. lithium potential of less than 3.7 V (Li/Li⁺), which isrelatively lower than that of the first lithium-containing metalcomposite oxide. Herein, the range of a potential difference vs. lithiumpotential (Li/Li⁺, ΔP) between the first lithium-containing metalcomposite oxide and the second lithium-containing metal composite oxideis within a range of 0.3 V<ΔP<5 V, and more preferably within a range of0.3V<ΔP<3.5V, but is not limited to this range.

An electric resistance of an object varies according to the shape, thesize, and the way of current flow (even in the case of objects using thesame material). For example, when the constant current flows between acouple of opposite surfaces of a 1 m³ cube, V is represented by V=ρ·Iaccording to Ohm's law (herein, V represents the voltage between thesurfaces, I represents the current, and ρrepresents the resistivity).The resistivity is expressed in Ω·m (as an MKSA unit). When an objecthas a cross sectional area of Sm², and a length of Lm, the resistance Ris represented by R=ρ·L/S.

In the present invention, it is possible to adjust the resistance value(R₂) of a second lithium-containing metal composite oxide, by adjustinga coated thickness (L) and a specific surface area (S) of the secondlithium-containing metal composite oxide. Accordingly, the secondlithium-containing metal composite oxide may not function as an internalelectric resistance layer during a normal discharge, but may function asan internal electric resistance layer during a high-rate discharge morethan a certain value (for example, 5 C). For example, as the coatedthickness (L) and the specific surface area (S) are thicker and smaller,respectively, the second lithium-containing metal composite oxide has ahigher resistance value (R₂). The specific surface area is adjusted byadjusting the size of the second lithium-containing metal compositeoxide.

When the resistance value (R₂) of the second lithium-containing metalcomposite oxide is increased, the current value (I) passing through thecoated layer of the second lithium-containing metal composite oxide isdecreased because of V=IR (V: voltage, I: current, and R: resistance).Therefore, when heating value (Q/t=VI, Q=heating value, t=time) per unittime (t) is decreased in the core particle of a first lithium-containingmetal composite oxide, the safety of a battery can be improved.

The range of a current rate, in which a coated layer of the secondlithium-containing metal composite oxide acts as a significant electricresistance layer, is not limited as long as the current is out of therange of a normal battery operation. For example, the range may be 5 Cor more.

Further, a resistance difference between a first lithium-containingmetal composite oxide and a second lithium-containing metal compositeoxide may be represented in the electrical conductivity ratio of thesecond lithium-containing metal composite oxide to the firstlithium-containing metal composite oxide. Herein, the electricalconductivity ratio preferably has a range of 10⁻¹˜10⁻⁷, and morepreferably has a range of 10⁻¹˜10⁻³, but is not limited to this. Whenthe resistance difference between the two materials has a large value,the battery safety is improved, but the battery performance may bereduced, and on the other hand, when the resistance difference has asmall value, it is difficult to achieve a desired effect of the presentinvention. Since during an internal short (that is, discharge), lithiumions prefer to be intercalated in the first lithium-containing metalcomposite oxide having higher potential, the second lithium-containingmetal composite oxide coated on the first lithium-containing metalcomposite oxide inevitably acts as a resistance.

Also, when the electrical conductivity ratio of the secondlithium-containing metal composite oxide to the first lithium-containingmetal composite oxide is less than the above described range, (that is,the resistance difference is large), it is possible to coat the surfaceof the second lithium-containing metal composite oxide with a conductivematerial (for example, a carbonaceous material, etc.) in order to adjustthe ratio to the above described range.

Non-limiting examples of the first lithium-containing metal compositeoxide that may be used in the present invention include LiMO₂(M=Co, Mn,Ni, Ni_(1/3)Co_(1/3)Mn_(1/3), Cr, V), LiMo₄(M=CoMn, NiV, CoV, CoP, MnP,NiP, Mn₂) and a mixture of the materials, and non-limiting examples ofthe second lithium-containing metal composite oxide include LiMo₄(M=V₂,FeP), LiMo₂(M=Mn, Mo, W), LiV₆O₁₃, LiTiS₂, LiWO₂ and a mixture of thematerials. In addition, a reversible compound capable of intercalatingand deintercalating lithium, which has a potential difference and aresistance ratio as described above, thereby improving the safety of abattery, may be used in the present invention.

In the cathode active material according to the present invention, it ispreferable that particles of a second lithium-containing metal compositeoxide having a relatively low potential vs. lithium potential and a highresistance are covered on the entire surface of a firstlithium-containing metal composite oxide (see FIG. 1). In order toobtain such a cathode active material, the surface area, size, and usagerate of a first lithium-containing metal composite oxide and a secondlithium-containing metal composite oxide are required to be adjusted. Acathode according to the present invention will be obtained by adjustingthe surface area, size, and usage rate of a second lithium-containingmetal composite oxide.

For example, in order to coat a first lithium-containing metal compositeoxide particle having a particle size of 20 μm, a secondlithium-containing metal composite oxide (particle size: 5 μm) isrequired to be used in an amount of about 20 wt %. On the other hand,when a particle size of a second lithium-containing metal compositeoxide is 2.5 μm, the first lithium-containing metal composite oxide maybe completely coated with an amount of 7.5 wt % of the secondlithium-containing metal composite oxide.

Therefore, in the present invention, in order to obtain a cathode activematerial having a coated surface as described above, the surface area,particle size, and usage rate of the second lithium-containing metalcomposite oxide may be appropriately adjusted within a conventionalrange known to one skilled in the art. Herein, the weight ratio of thefirst lithium-containing metal composite oxide to the secondlithium-containing metal composite oxide is preferably within the rangeof 70:30˜97.5:2.5, but is not limited to this. When the weight ratio isless than the above range, electrochemical performance may be reduced.On the other hand, when the weight ratio is more than the above range,the second lithium-containing metal composite oxide cannot sufficientlyact as a resistance, and thus the safety of a battery cannot beimproved.

However, when there is a large difference between specific surface areasof the first lithium-containing metal composite oxide and the secondlithium-containing metal composite oxide, it is possible to achieve thesame effect as that of the above described range although the weightratio is smaller than that of the range. Also, contrary to simplyblended two active materials in which the weight ratio is required to beincreased due to unmixed and conglomerated particles, when the surfaceof either particle is coated with another material as in the presentinvention, it is possible to achieve the same effect as that of theabove described range although the weight ratio is smaller than that ofthe range.

As described above, in order to coat the surface of a firstlithium-containing metal composite oxide with a secondlithium-containing metal composite oxide, it is preferable that thefirst lithium-containing metal composite oxide has a particle sizewithin the range of 5 μm˜30 μm, and the second lithium-containing metalcomposite oxide has a particle size within the range of 30 nm˜5 μm.Also, it is preferable that the first lithium-containing metal compositeoxide has a specific surface area within the range of 0.01˜0.2 m²/g, andthe second lithium-containing metal composite oxide has a specificsurface area within the range of 0.5˜30 m²/g. But, the particle size andthe specific area are not limited to the above described ranges.

For example, in the case of LiCoO₂ used as a first lithium-containingmetal composite oxide, the particle size may be within the range of10˜20 μm, the electrical conductivity may be about 10⁻² S/cm, and thespecific surface area may be about 0.2 m²/g. As a secondlithium-containing metal composite oxide to be coated on the surface ofthe above described particle, LiMnO₄, Li(Ni_(1/3)Mn_(1/3)Co_(1/3))O₂,LiFePO₄, etc. may be used. In the case of LiFePO₄ used as a secondlithium-containing metal composite oxide, the particle size may bewithin the range of 0.5˜5 μm, the electrical conductivity may be about10⁻⁴˜10⁻⁹ S/cm, and the specific surface area may be about 0.5˜13 m²/g.When the whole or partial surface of LiFePO₄ (the secondlithium-containing metal composite oxide) is coated with a carbonaceousmaterial, the conductivity may be increased up to the range of10⁻²˜10⁻³.

Non-limiting examples of a method of obtaining a cathode active materialaccording to the present invention (in which the surface of a firstlithium-containing metal composite oxide is coated with a secondlithium-containing metal composite oxide) include jet mill, spraydrying, etc. Preferably, a Mechanofusion method may be used, and also, aNOBILTA™ (Hosokawa Micron Corporation), which has better milling powerthan the Mechanofusion method, may be used.

When a first lithium-containing metal composite oxide and a secondlithium-containing metal composite oxide are charged in a Mechanofusionmixer, the materials are mixed by adding mechanical energy and thermalenergy at the same time. Accordingly, it is possible to obtain a cathodeactive material in which the surface of a first lithium-containing metalcomposite oxide particle is uniformly coated with a secondlithium-containing metal composite oxide.

In a Mechanofusion method, when a high striking energy is added toparticles, two kinds of particles are fused in such a manner that a baseparticle is completely surrounded by coating particles. For example,Such a coated particle is obtained by the steps of: fixing both kinds ofpowder on the inner wall of a rotative container by a centrifugal force;momentarily compacting the powder by an inner member fixed to a centershaft; scraping the powder with a scraper at a high speed; complexingthe particles through repeated compacting/scraping(shearing) operationand heat-adhering both kinds of powder by a mechanofusion operation.However, this example is illustrative only, and the scope of the presentinvention is not limited thereto.

In the present invention, particle hardness of the secondlithium-containing metal composite oxide may be lower than that of thefirst lithium-containing metal composite oxide because, in the processof coating the second lithium-containing metal composite oxide on thesurface of the first lithium-containing metal composite oxide by millingthe two materials, the second lithium-containing metal composite oxidehas to be finely ground so as to be uniformly coated on the surface ofthe first lithium-containing metal composite oxide. On the other hand,the first lithium-containing metal composite oxide does not have to beground.

A cathode including the cathode active material according to the presentinvention is obtained by applying cathode slurry including the cathodeactive material according to the present invention on a currentcollector, and drying the applied slurry. Herein, a small amount ofconductive material or binder may be optionally added.

Non-limiting examples of a cathode current collector include aluminum,nickel, and foil obtained by combination of aluminum and nickel. As abinder, a conventional binding agent may be used, and non-limitingexamples of the binder include PVDF (polyvinylidene fluoride), SBR(styrene butadiene rubber), etc.

The present invention provides a secondary battery including a cathodeincluding a first cathode active material surface-coated with a secondcathode active material, an anode, electrolyte, and a separator, thesecond cathode active material having a relatively low potential and ahigh resistance.

It is preferable that the secondary battery is a lithium secondarybattery, and non-limiting examples of the lithium secondary batteryinclude a lithium metal secondary battery, a lithium ion secondarybattery, a lithium polymer secondary battery, a lithium ion polymersecondary battery, etc.

The secondary battery according to the present invention may be obtainedby a conventional method known to one skilled in the art in which aporous separator is inserted between a cathode and an anode, andelectrolyte is added.

An anode according to the present invention may be obtained by aconventional method known to one skilled in the art, that is, byapplying anode slurry including an anode active material on a currentcollector, and drying the applied slurry. Herein, the anode activematerial may include a conventional anode active material that may beused in an anode of a conventional secondary battery, and non-limitingexamples of the anode active material include lithium metal or lithiumalloy, a lithium intercalating material, such as carbon, petroleum coke,activated carbon, graphite, or other carbons, etc.

The electrolyte for the battery includes conventional electrolytecomponents known to one skilled in the art, that is, electrolyte saltsand an organic solvent.

An electrolyte salt that may be used in the present invention includes asalt represented by the formula of A⁺B⁻, wherein A⁺ represents an alkalimetal cation selected from the group including Li⁺, Na⁺, K⁺ andcombinations thereof, and B⁻ represents an anion selected from the groupincluding PF₆ ⁻, BF₄ ⁻, Cl⁻, Br⁻, I⁻, ClO₄ ⁻, AsF₆ ⁻, CH₃CO₂ ⁻, CF₃SO₃⁻, N(CF₃SO₂)₂ ⁻, C(CF₂SO₂)₃ ⁻ and combinations thereof. A lithium saltis particularly preferred.

As an organic solvent, a conventional solvent known to one skilled inthe art, that is, cyclic carbonate and/or linear carbonate, may be used,and non-limiting examples of such carbonate include propylene carbonate(PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethylcarbonate (DMC), dipropyl carbonate (DPC), dimethyl sulfoxide,acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran,N-methyl-2-pyrrolidone (NMP), ethyl methyl carbonate (EMC),gamma-butyrolactone (GBL), fluoroethylene carbonate (FEC), methylformate, ethyl formate, propyl formate, methyl acetate, ethyl acetate,propyl acetate, pentyl acetate, methyl propionate, ethyl propionate,propyl propionate, butyl propionate, or a mixture thereof. Halogenderivatives of the above organic solvents may also be used.

There is no particular limitation in the separator that may be used inthe present invention, as long as the separator is a porous material.Non-limiting examples of the separator that may be used include apolypropylene-based, polyethylene-based, polyolefin-based porousseparator. Porous separators to which inorganic materials are added mayalso be used.

There is no particular limitation in the outer shape of the secondarybattery according to the present invention. The secondary battery may bea cylindrical battery using a can, a prismatic battery, a pouch-typebattery or a coin-type battery.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a scanning electron microscopy (SEM) picture of an activematerial LiCoO₂ surface-coated with LiFePO₄, which is prepared byExample 1;

FIG. 2 is a SEM picture of an active material LiCoO₂ simply mixed withLiFePO₄, which is prepared by Comparative Example 2;

FIG. 3 is a schematic drawing illustrating active materials according toExamples and Comparative Examples;

FIGS. 4˜8 illustrate DC impedance values of an active material LiCoO₂surface-coated with LiFePO₄, which is prepared by Example 4, and DCimpedance values of an active material LiCoO₂, which is prepared byComparative Example 1. FIG. 4, FIG. 5, FIG. 6, FIG. 7 and FIG. 8illustrate the values at the current rates of 1 C, 5 C, 10 C, 20 C, and30 C, respectively.

FIG. 9 is a SEM picture of an active material LiCoO₂ surface-coated withLiFePO₄, which is prepared by Example 5.

BEST MODE FOR CARRYING OUT THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention. However, the following examples are illustrativeonly, and the scope of the present invention is not limited thereto.

EXAMPLE 1

LiCoO₂ (specific surface area: 0.2 m²/g, D₅₀=5˜20 μm) and LiFePO₄(specific surface area: 13 m²/g, D₅₀=0.5˜5 μm) were mixed in a weightratio of 97.5:2.5, and the mixture was charged into a mechanofusionmixer. A mechanofusion process was performed for 5˜30 minutes to obtainLiCoO₂ powder surface-coated with LiFePO₄. Although the mechanofuxionprocess was performed at room temperature, the process inevitablygenerated considerable amount of heat. So, the coating of LiFePO₄ isfacilitated by the heat. FIG. 1 shows a scanning electron microscopy(SEM) picture of the obtained powder. Herein, the potential differencevs. lithium potential between the two materials was 0.3V˜0.8V, and theelectrical conductivity ratio of LiFePO₄ to LiCoO₂ was 10⁻²˜10⁻⁷.

As shown in FIG. 1, fine particles of LiFePO₄ are uniformly coated onthe surface of LiCoO₂ particles.

The coated cathode active material, together with a conductive materialand a binding agent, was added in a solvent NMP(N-methyl-2-pyrrolidone), to obtain cathode slurry. Then, a cathode wasobtained by coating the cathode slurry on Al current collector.

As an anode active material, a graphite material was used. A bindingagent was added in NMP to obtain anode slurry. Then, an anode wasobtained by coating the anode slurry on Cu current collector.

As electrolyte, EC/EMC (ethyl carbonate/ethyl methyl carbonate,volumetric ratio 1:2) solution containing 1M LiPF₆ dissolved therein wasused. A polyolefin-based separator was inserted between the obtainedcathode and anode, and then the electrolyte was charged to obtain abattery.

EXAMPLE 2

A cathode active material and a battery were obtained in the same manneras described in Example 1, except that LiCoO₂ and LiFePO₄ were mixed ina weight ratio of 95:5 instead of 97.5:2.5.

EXAMPLE 3

A cathode active material and a battery were obtained in the same manneras described in Example 1, except that LiCoO₂ and LiFePO₄ were mixed ina weight ratio of 90:10 instead of 97.5:2.5.

EXAMPLE 4

A cathode active material and a battery were obtained in the same manneras described in Example 1, except that LiCoO₂ and LiFePO₄ were mixed ina weight ratio of 85:15 instead of 97.5:2.5.

EXAMPLE 5

A cathode active material and a battery were obtained in the same manneras described in Example 1, except that a NOBILTA™ (Hosokawa MicronCorporation) was performed instead of a mechanofusion method.

COMPARATIVE EXAMPLE 1

A cathode active material and a battery were obtained in the same manneras described in Example 1, except that LiCoO₂ was used alone as acathode active material.

COMPARATIVE EXAMPLE 2

A cathode active material and a battery were obtained in the same manneras described in Example 3 (LiCoO₂:LIFePO₄=90:10), except that LiCoO₂ andLiFePO₄ were simply mixed by a ball mill process, instead of amechanofusion process.

COMPARATIVE EXAMPLE 3

A cathode active material and a battery were obtained in the same manneras described in Example 1, except that LiNi_(0.9)Co_(0.1)O₂ (specificsurface area: 0.4 m²/g, D₅₀=10 μm), instead of LiFePO₄, was coated onthe surface of LiCoO₂ via a mechanofusion process.

Herein, the potential difference vs. lithium potential between the twomaterials was −0.2V˜0.5V, and the electrical conductivity ratio ofLiNi_(0.9)Co_(0.1)O₂ to LiCoO₂ was 1.

When the secondary battery according to Comparative Example 3 was used,the safety of a battery cannot be improved due to an inappropriateresistance ratio.

EXPERIMENTAL EXAMPLE 1 Determination on the Safety of a Battery

On each of the secondary batteries according to Examples 1˜4 and thebatteries according to Comparative Examples 1˜3, a nail penetration testwas performed with voltage of 4.2V at 60° C.

As noted in Table 1, batteries according to Comparative Examples 1˜3caused fire, and on the other hand, batteries according to Examples 1˜4did not cause fire or explosion, and maintained safety. Therefore,although an internal short was caused by an external impact, it ispossible to significantly reduce the momentary movement of a largeamount of lithium ions, and the flow of current, from an anode to acathode. In other words, heat accumulation of a battery, which is causedby the flow of current, is reduced, thereby suppressing sudden fire orexplosion of a battery.

TABLE 1 0.3 m/min 1 m/min 3 m/min Exp. 1 Pass Pass Pass (0/3) (0/3)(0/3) Exp. 2 Pass Pass Pass (0/3) (0/3) (0/3) Exp. 3 Pass Pass Pass(0/3) (0/3) (0/3) Exp. 4 Pass Pass Pass (0/3) (0/3) (0/3) Comp. Exp.Fail Fail Fail 1 (3/3) (3/3) (3/3) Comp. Exp. Fail Fail Fail 2 (3/3)(3/3) (3/3) Comp. Exp. Fail Fail Fail 3 (3/3) (3/3) (3/3)

EXPERIMENTAL EXAMPLE 2 Determination on the Resistance of a Battery

On each of the secondary battery according to Example 1 and the batteryaccording to Comparative Example 1, an internal resistance (DCimpedance) was measured by changing the current rate into 1 C, 5 C, 10C, 20 C, and 30 C.

Based on the test results, as the current rate is increased, thedifference in DC impedance between LiCoO₂ surface-coated with LiFePO₄and uncoated LiCoO₂ gets larger (see FIGS. 4˜8). Accordingly, it isdetermined that during discharge, LiFePO₄ acting as a secondlithium-containing metal composite oxide functions as a resistance.

Therefore, it is determined that during a discharge of 5 C or more (forexample, an internal short of a battery) in which the resistancedifference of two or more kinds of cathode active materials occurs, themoving rate of a large amount of lithium ions and electrons from ananode to a cathode is decreased, thereby preventing the heat generationcaused by the sudden occurrence of over current, thus improving thesafety of a battery.

EXPERIMENTAL EXAMPLE 3 Determination on the Thermal Stability of aBattery

The thermal stability of a battery was determined by using DifferentialScanning calorimetry (DSC), and the performance of a battery wasdetermined by manufacturing a coin half cell.

TABLE 2 Comp. Comp. Exp. 1 Exp. 1 Exp. 2 Sample weight (mg) 9.0 9.0 9.0Relative weight (%) of 100 97.5 90 LiCoO₂ Heat value (J/g) 384.2 356.6385 Reversible 0.1 C 147.3 145.5 138.6 capacity 0.2 C 146.0 143.0 136.0(charge/ 0.2 C/ 99.1 98.3 98.1 discharge) 0.1 C (mAh/g) 0.5 C 142.1139.0 131.7 0.5 C/ 96.5 95.5 95.1 0.1 C   1 C 136.4 132.3 125.8   1C/0.1 C 92.6 90.9 90.8 1.5 C 132.8 129.1 120.1 1.5 C/ 90.2 88.7 86.7 0.1C   2 C 122.1 118.4 105.1   2 C/0.1 C 82.9 81.3 75.8

Based on the heat values as noted in Table 2, a surface-coated sampleaccording to Example 1 has a lower value than an uncoated sampleaccording to Comparative Example 1. Therefore, it is determined that thethermal stability of an active material of the battery according toExample 1 is better. On the other hand, the thermal stability of thebattery according to Comparative Example 2, which is simply mixedsample, is not good.

Also, as noted in Table 2, at a low-rate, a surface-coated sampleaccording to Example 1 shows similar performance to an uncoated sampleaccording to Comparative Example 1. Therefore, it is determined that anactive material according to the present invention may improve thestability while minimizing the reduction of battery performance. On theother hand, in the case of simply mixed sample according to ComparativeExample 2, since the content of LiCoO₂ is decreased, the loss ofcapacity is unavoidable.

In the case of high-rates such as 5 C, 10 C, 30 C, etc. (not noted inTable 2), as shown in FIGS. 4˜8, since the resistance is increased, theperformance of a battery is reduced and the moving rate of electrons andlithium ions is reduced. Therefore, it is possible to improve the safetyat the occurrence of over current.

Industrial Applicability

In the present invention, an entire surface of a firstlithium-containing metal composite oxide is coated with a secondlithium-containing metal composite oxide having a high resistance valueand a low potential vs. lithium potential. Therefore, during an internalshort of a secondary battery, it is possible to slow down the movingrate of a large amount of lithium ions and electrons from an anode to acathode, and thus to prevent heat generation caused by the occurrence oftemporary over-current. Also, it is possible to increase the temperaturewhere a cathode active material is decomposed and gas is generated, andto improve thermal stability by inhibiting side reactions of the cathodeactive material and electrolyte.

Also, since the second lithium-containing metal composite oxide iscoated on the entire surface of the first lithium-containing metalcomposite oxide, a small amount of second lithium-containing metalcomposite oxide is required. Accordingly, it is possible to minimize thedegradation of battery capacity and performance.

While this invention has been described in connection with what ispresently considered to be the most practical and exemplary embodiment,it is to be understood that the invention is not limited to thedisclosed embodiment and the drawings, but, on the contrary, it isintended to cover various modifications and variations within the spiritand scope of the appended claims.

1. A cathode active material comprising: (a) a first lithium-containingmetal composite oxide; and (b) a coating layer comprising a secondlithium-containing metal composite oxide coated on an entire surface ofthe first lithium-containing metal composite oxide particle, the coatinglayer being free from a carbonaceous material, wherein the secondlithium-containing metal composite oxide having a higher resistance anda lower potential vs. lithium potential (Li/Li⁺) than the firstlithium-containing metal composite oxide, and wherein the firstlithium-containing metal composite oxide has a specific surface areawithin a range of 0.01˜0.2 m²/g, and the second lithium-containing metalcomposite oxide has a specific surface area within a range of 0.5˜30m²/g.
 2. The cathode active material according to claim 1, wherein thefirst lithium-containing metal composite oxide has a potential vs.lithium potential of 3.7V or more (Li/Li⁺), and the secondlithium-containing metal composite oxide has a potential vs. lithiumpotential of less than 3.7V (Li/Li⁺).
 3. The cathode active materialaccording to claim 1, wherein a potential difference vs. lithiumpotential (ΔP) between the first lithium-containing metal compositeoxide and the second lithium-containing metal composite oxide is withina range of 0.3V<ΔP<5V.
 4. The cathode active material according to claim1, wherein an electrical conductivity ratio of the secondlithium-containing metal composite oxide to the first lithium-containingmetal composite oxide is within a range of 10⁻¹˜10⁻⁷.
 5. The cathodeactive material according to claim 1, wherein a weight ratio of thefirst lithium-containing metal composite oxide to the secondlithium-containing metal composite oxide is within a range of70:30˜97.5:2.5.
 6. The cathode active material according to claim 1,wherein the first lithium-containing metal composite oxide has aparticle size within a range of 5˜30 μm, and the secondlithium-containing metal composite oxide has a particle size within arange of 30 nm˜5 μm.
 7. The cathode active material according to claim1, wherein the first lithium-containing metal composite oxide particleis selected from the group consisting of LiMO₂ (M=Co, Mn, Ni,Ni_(1/3)Co_(1/3)Mn_(1/3), Cr, V) and LiMO₄ (M=CoMn, NiV, CoV, CoP, MnP,NiP, or Mn₂), and the second lithium-containing of LiMO₄ (M=V₂, OR FeP),LiMO₂ (M=Mn, Mo, or W), LiV₆O₁₃, LiTiS₂, and LiWO₂.
 8. The cathodeactive material according to claim 1, wherein the secondlithium-containing metal composite oxide has a lower particle hardnessthan the first lithium-containing metal composite oxide.
 9. A secondarybattery comprising a cathode, an anode, electrolyte, and a separator,wherein the cathode comprises the cathode active material according toclaim
 1. 10. The secondary battery according to claim 9, wherein duringa discharge of 5 C or more, a moving rate of a large amount of lithiumions and electrons, which is suddenly transferred from said anode tosaid cathode, is adjusted to slowed down, thereby preventing fireoccurrence of the battery.
 11. The secondary battery according to claim9, wherein the secondary battery is a lithium secondary battery.